The present invention relates to compositions of lithographic printing inks and lithographic printing methods.
Printing inks generally include one or more vehicle polymers or resins and one or more colorants as principal components. Printing ink vehicles must meet a number of performance requirements that include both requirements related to the printing process, such as suitable consistency and tack for sharp, clean images, suitable length to avoid fly or mist, or proper drying characteristics, and requirements related to the printed image, such as gloss, chemical resistance, durability, or color. In general, ink vehicles include one or more materials such as vegetable oils or fatty acids, resins, and polymers that contribute to the end product properties, and may include other components such as organic solvents, water, rheology modifiers, and so on that may affect body, tack, or drying characteristics.
In lithographic printing, in particular, an inked printing plate contacts and transfers an inked image to a rubber blanket, and then the blanket contacts and transfers the image to the surface being printed. Lithographic plates are produced by treating the image areas of the plate with an oleophilic material and ensuring that the non-image areas are hydrophilic. In a typical lithographic printing process, the plate cylinder first comes in contact with dampening rollers that transfer an aqueous fountain solution to the hydrophilic non-image areas of the plate. The dampened plate then contacts an inking roller, accepting the ink only in the oleophilic image areas. The press operator must continually monitor the printing process to insure that the correct balance of the fountain solution and the ink is maintained so that the ink adheres to the printing areas, but only the printing areas, of the plate in order to produce a sharp, well-defined print.
The industry has long sought an offset printing process and associated materials that would not require a separate fountain solution. Waterless plates have been made by applying to the non-image area a silicone rubber, which has a very low surface energy and is not wetted by the ink. The silicone-modified plates are expensive, however, and require expensive, specially-cooled press equipment because the fountain solution of the traditional two-fluid method also serves as a coolant. Other efforts have been directed to producing a single-fluid lithographic ink, i.e., an ink that does not require a separate fountain solution, that can be used with the industry-standard presses and all-metal plates. Parkinson, in U.S. Pat. No. 4,045,232 (the entire disclosure of which is expressly incorporated herein by reference) describes lithographic printing and earlier efforts directed to producing a single-fluid lithographic ink and the tendency of single-fluid inks to be unstable. Parkinson notes that ink emulsions containing a solution of glycerin and salts tend to xe2x80x9cbreak,xe2x80x9d with the result that the glycerin wets the inking rollers preventing good inking. Parkinson suggests an improved single-fluid ink obtained by using an additive that includes a resin treated with a concentrated mineral acid, and, optionally, a polyhydric or monohydric alcohol. Preferred polyols are glycerin, ethylene glycol, and propylene glycol. DeSanto, Jr. et al, in U.S. Pat. No. 4,981,517 (the entire disclosure of which is expressly incorporated herein by reference) describe a printing ink that is an emulsion of an oil-based phase and a water-miscible phase. The patentees allege that an emulsion containing a significant portion of water (10% to 21%) and employing phosphoric acid as a critical component has improved stability against phase separation and can be used as a single-fluid lithographic ink. The De Santo, Jr. composition further includes as a diluent and emulsion stabilizer an oil with the properties of No. 1 and No. 2 fuel oils and a polyol emulsifier, of which glycerin and ethylene glycol are the only examples provided.
Nonetheless, due to various drawbacks of the single-fluid lithographic inks that have previously been proposed, including the limited stability and poor definition and toning already mentioned, the industry standard continues to be a dual-fluid lithographic ink that includes an ink component and a separate fountain solution component.
The invention provides a single fluid lithographic printing ink composition that includes a hydrophobic phase and an emulsified hydrophilic fluid phase. The hydrophobic phase comprises a hydrogen bonding vinyl resin. The terms xe2x80x9cvinyl resinxe2x80x9d and xe2x80x9cvinyl polymerxe2x80x9d are used synonymously in describing the invention and refer to polymers prepared by chain reaction polymerization, or addition polymerization, through carbon-carbon double bonds, using vinyl monomers (such as acrylic and methacrylic monomers, other vinyl monomers including vinyl esters and vinyl aromatic monomers including styrene) and other ethylenically unsaturated monomers copolymerizable with these. The term xe2x80x9chydrogen bondingxe2x80x9d vinyl polymers refers to those vinyl polymers having groups that form hydrogen bonds with the emulsified hydrophilic fluid phase.
The vinyl polymers of the invention are preferably branched by including in the polymerization reaction monomers that have two or more, preferably two reaction sites or by reacting the polymer with a limited amount of crosslinker during or after polymerization. The xe2x80x9cbranchedxe2x80x9d vinyl polymer is one that remains usefully soluble in organic solvents, rather than a polymer that is crosslinked into an insoluble, three-dimensional network structure that can only be swelled by solvents. The branched vinyl polymers of the invention unexpectedly retain solubility in spite of significant branching.
The invention further provides a method of making an ink composition having a hydrophilic fluid phase and a hydrophobic phase that includes an organic solution of vinyl resin that can form hydrogen bond interactions with the hydrophilic fluid phase. In another aspect of the invention, the hydrophobic phase includes one or more other additional resins or polymers.
The invention also provides a process of printing using the lithographic single fluid ink of the invention. The invention has unexpectedly provided stable inks that can be used as single fluid inks with excellent fountain stability and resistance to toning.